PSA and Free PSA Are Measured in Serum
Both prostate-specific antigen (PSA) titer and free PSA titer are measured in serum (blood), not urine. This is the established standard in clinical practice for prostate cancer screening, diagnosis, and monitoring.
Clinical Standard for PSA Measurement
The clinical guidelines consistently specify serum PSA measurement as the standard approach:
- Serum PSA should be measured from patients presenting with urinary symptoms 1
- Serum total PSA is used for routine staging and risk assessment 1
- Measuring serum free PSA may help assess prostate cancer risk, especially when serum total PSA is in the 4-10 ng/mL range 1
- The AUA guideline explicitly recommends measurement of the serum prostate-specific antigen (PSA) in select patients 2
Why Serum and Not Urine?
Molecular Forms in Serum
PSA circulates in blood in two main forms 3, 4:
- Free PSA (fPSA): unbound PSA with molecular weight allowing kidney filtration
- Complexed PSA (cPSA): PSA bound to alpha-1-antichymotrypsin (~90 kDa), too large for kidney filtration
Clearance Pathways
- Free PSA is cleared by the kidneys (half-life <1 hour) 3
- Complexed PSA is cleared by the liver (half-life 2-3 days) 3, 4
- Total PSA measured in serum represents the combination of free and complexed forms 3
Urine PSA Is Not Clinically Validated
Research evidence demonstrates that urine PSA values do not reliably reflect serum PSA values 5. While PSA can be detected in urine, the correlation with serum values is weak (r < 0.24), making urine measurements unsuitable for clinical decision-making 5. Urinary PSA levels do not correlate with serum levels and cannot be used interchangeably 6, 7.
Clinical Reference Ranges
Serum-based reference ranges are well-established 1, 2:
- 0-2 ng/mL: 1% probability of prostate cancer
- >10 ng/mL: >50% probability of prostate cancer
- 4-10 ng/mL: "diagnostic gray zone" where free PSA ratio is most useful
The free-to-total PSA ratio in serum improves specificity, with values <0.25 suggesting higher cancer risk 8, 9.
Important Caveats
Factors Affecting Serum PSA
Multiple factors can alter serum PSA levels 10:
- Elevate PSA: prostatitis, BPH, urethral/prostatic trauma, prostate cancer
- Lower PSA: finasteride/dutasteride (reduce by ~50%), androgen suppression
- Minimal effect: ejaculation, digital rectal examination (variable/insignificant)
- Significant elevation: prostate biopsy (wait 3-6 weeks before testing)
Laboratory Considerations
- Assay variability: 20-25% depending on standardization method 10
- Use same assay for longitudinal monitoring—assays are not interchangeable 10
- Confirm abnormal results before proceeding to biopsy 10
Special Populations
- Hemodialysis patients: Total serum PSA remains valid, but free PSA interpretation requires caution due to altered clearance 3
- End-stage renal disease: Total PSA remains useful; free PSA and %fPSA may be affected 3
Bottom line: Order serum PSA and serum free PSA for clinical decision-making. Urine PSA has no validated role in prostate cancer screening or diagnosis.